The present invention relates generally to data storage. More specifically, a multilevel sequence is described that is used for calibration of a system that precompensates for intersymbol interference.
In order to increase the capacity and speed of optical data storage systems, multilevel optical recording systems have been developed. In a traditional optical recording system, reflectivity of the recording medium is modulated between two states. The density of data recorded on an optical recording medium can be increased by modulating the reflectivity of the optical recording medium among more than two states. Multilevel optical recording refers to recording multiple levels of data (i.e. more than two levels of data) at a location on an optical storage medium.
One type of optical recording medium that is particularly suitable for multilevel signal modulation is phase-change optical material. The reflectivity of a phase-change material may be controlled by using a writing laser to heat and cool the material. This process is described further in xe2x80x9cLaser-induced crystallization phenomena in GeTe-based alloys: Characterization on nucleation and growth, (J. Appl. Phys. 78(8), Oct. 15, 1995. p.4096) by J. H. Coombs, et al. (hereinafter xe2x80x9cCoombsxe2x80x9d) and in U.S. patent application Ser. No. 09/373,916, filed Aug. 12, 1999, entitled xe2x80x9cHigh Density Data Write Strategyxe2x80x9d which is incorporated herein by reference for all purposes. Multilevel data written on a phase-change optical disc is recovered by measuring the intensity of a beam of light reflected from the disc.
Unfortunately, at high data densities, there is a significant amount of linear and nonlinear intersymbol interference (ISI) in the reflected signal from such a disc. One technique for reducing the nonlinear ISI is to adjust the way marks are written in order to precompensate during the writing process for an expected amount of nonlinear ISI. Precompensation for ISI is described in detail in U.S. Pat. No. 5,818,806, issued Oct. 6, 1998, entitled xe2x80x9cMethod And Apparatus For Providing Equalization For The Reading Of Marks On Optical Data Storage Mediaxe2x80x9d. Different combinations of multilevel symbols have different amounts of linear and nonlinear ISI. The amount of ISI, especially the nonlinear ISI, is difficult to predict and may vary for different systems designed within the same manufacturing tolerances. In fact, the ISI may vary for a system as that system experiences wear.
In order to determine the appropriate amount of precompensation for a specific system at a given time, a calibration sequence can be written to and read from the recording media. By analyzing the detected output of such a calibration sequence, it is possible, when the input sequence is known, to determine an appropriate precompensation scheme that will compensate for intersymbol interference. One such precompensation scheme is described in U.S. patent application Ser. No. 09/496,895 entitled xe2x80x9cWrite Compensation System for a Multi-Level Data Storage Systemxe2x80x9d filed concurrently herewith, which is incorporated herein by reference for all purposes.
What is needed is an effective method for generating a calibration sequence containing all possible subsequences of a given length for a set of multilevel symbols. Furthermore, it would be desirable for such a calibration sequence to be pseudorandom instead of having pseudo-periodicities. Specifically, it is important that the calibration sequence not contain a high spectral content at low frequencies compared its spectral content at high frequencies. Also, it would be useful if such a system could use as little memory as possible.
Accordingly, a method is disclosed for generating a minimal length calibration sequence containing all subsequences of a given length for a set of multilevel symbols. The calibration sequence is pseudorandom, and therefore has a flat frequency spectrum. One method uses a maximal-length shift register along with some additional logic to generate the calibration sequence. In some embodiments, a sequence is modified to provide DC control while still preserving the occurrence of all combinations of subsequences. In this manner, low frequency spectral components in the sequence spectrum are suppressed. In some embodiments, a minimal length calibration sequence is converted into another sequence that is not minimal length for the purpose of suppressing low frequency spectral content. The calibration sequence can be used to determine a precompensation scheme for data written to and read from a physical medium, such as magnetic or optical data storage disks.
It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer-readable medium that includes certain types of marks that enable reliable data storage and recovery. Several inventive embodiments of the present invention are described below.
In one embodiment, a method of generating a calibration sequence of multilevel symbols includes generating subsequences of digital values selected from a range of digital values using a maximal length shift register. The subsequences have a given subsequence length. The range of digital values is mapped onto a set of multilevel symbols and the multilevel symbols corresponding to the generated digital values are output to a write strategy processor.
In one embodiment, a calibration sequence generator includes a maximal length shift register configured to generate a calibration sequence of multilevel symbols by generating subsequences of digital values selected from a range of digital values. The subsequences have a given subsequence length. A symbol generator is configured to map the range of digital values onto a set of multilevel symbols and to output the multilevel symbols corresponding to the generated digital values to a write strategy processor.
In one embodiment, a computer program product for generating a calibration sequence of multilevel symbols is embodied in a computer readable medium and includes computer instructions for generating subsequences of digital values selected from a range of digital values using a maximal length shift register. The subsequences have a given subsequence length. Interactions are also included for mapping the range of digital values onto a set of multilevel symbols; and outputting the multilevel symbols corresponding to the generated digital values to a write strategy processor.
In one embodiment, a method of generating a calibration sequence of multilevel symbols includes generating subsequences of digital values selected from a range of digital values. The subsequences have a given subsequence length. The range of digital values is mapped onto a set of multilevel symbols and the multilevel symbols corresponding to the generated digital values are output to a write strategy processor.
In one embodiment, a calibration sequence generator is configured to generate a calibration sequence of multilevel symbols by generating subsequences of digital values selected from a range of digital values. The subsequences have a given subsequence length. A symbol generator is configured to map the range of digital values onto a set of multilevel symbols and to output the multilevel symbols corresponding to the generated digital values to a write strategy processor.